1. Field of the Invention
The present invention relates to a display apparatus in which a large number of light emitting elements as display pixels are arranged, for example, in a matrix pattern, and in particular to a display apparatus having a compensation means for compensating for over-time changes etc. of the above-mentioned light emitting elements, and a method of inspecting the display apparatus.
2. Description of the Related Art
Since mobile phones, mobile information terminals (PDA), etc., are widespread, there is an increasing demand for a display panel which has a high definition image display function and can realize a thin shape and low power consumption. Thus, conventionally, a liquid crystal display panel has been employed in a large number of products as a display panel which fulfils the demand.
On the other hand, a display panel has been recently put into practical use by using an organic EL (electroluminescence) element which takes advantage of a characteristic of being a self-emitting type element, thus attracting attention as a next-generation display panel which replaces the conventional liquid crystal display panel. There is also part of the background that a light-emitting layer of the element employs an organic compound which can expect a good light-emission property, so that the organic EL display panel has as high an efficiency and long a lifetime as can be put into practical use.
The above-mentioned organic EL element equivalently exhibits a diode characteristic. When a forward voltage is applied which is equal to or greater than a constant voltage (light-emission threshold voltage =Vth) inherent to an element, light is emitted. It is known that light-emission brightness at this time is substantially proportional to a value (an amount) of current which flows through the element. On the other hand, as for the above-mentioned organic EL element, it is known that physical properties of the element change due to long term use and a forward voltage Vf becomes large. For this reason, the organic EL element causes a variation over time in which the brightness characteristic with respect to an applied voltage is lowered due to real operating time.
Furthermore, in a range where the value of the applied voltage is greater than that of the above-mentioned light-emission threshold voltage, the organic EL element has a characteristic that the greater the value of the applied voltage is, the larger the light-emission brightness is. However, the higher the temperature is, the smaller the light-emission threshold voltage is. Therefore, the EL element is in a state where it can emit light at a higher temperature with a smaller applied voltage. Thus, even if the same light-emittable voltage is applied, it has temperature dependency of the brightness that it is bright at a high temperature, and dark at a low temperature.
Especially, the above-mentioned variation over time and the temperature dependency appear considerably, when the EL element is driven at the constant voltage. This is because the forward impedance of the element changes with a total drive period and ambient temperature so that the current which flows through the EL element changes.
Then, apart from the EL element which is arranged at the display panel and performs luminescence display, the present applicant has already filed a patent application with respect to a display apparatus having an EL element for monitoring (for detection) which measures a forward voltage Vf, in which a drive voltage from a power supply unit for driving the display panel is controlled by using the forward voltage Vf obtained with the EL element for monitoring. This is disclosed in patent document 1 as shown below, etc.
[Patent Document 1] Japanese Patent Publication (KOKAI) No. 2005-107003
FIG. 1 shows a basic structure of the display apparatus as disclosed in the above-mentioned patent document, provided with a current source I1 for supplying constant current in a forward direction to a detection element Em, and a detection circuit 1 for detecting the forward voltage Vf produced at an anode terminal of the detection element Em at this time. The above-mentioned detection circuit 1 is constituted by a sample and hold circuit, for example. A voltage corresponding to the above-mentioned forward voltage Vf detected by this detection circuit 1 is applied as a control voltage to a booster circuit (DC-DC converter) 2 which uses a battery as a primary power supply, for example.
The above-mentioned booster circuit 2 controls a DC output value corresponding to the forward voltage Vf obtained by the detection element Em, and functions as the power supply unit of a display panel 3 in which a large number of display elements (represented by EL element) E1, E2, E3, . . . , are arranged as pixels. In this case, it is preferable that the above-mentioned detection element Em is formed simultaneously with film formation process for the display elements E1, E2, E3, . . . , at the display panel 3, thus being able to match both electric characteristics and also to be able to match both environmental temperatures under display operation.
According to the structure as shown in FIG. 1 disclosed in the above-mentioned patent document, the drive voltage is controlled which is supplied from the power supply unit (booster circuit 2) corresponding to the variation over time of EL element or the change of environmental temperature, and the change of the luminescence characteristics by the variation over time of the EL element etc. can be inhibited. Further, according to this structure, since the voltage value required for causing and driving the EL element to emit light can be always secured without providing the above-mentioned power supply unit with an excessive voltage margin, thus being able to improve the usage efficiency of the power supply.
Incidentally, as described above, according to the structure provided with the detection element, a problem arises in that a suitable drive voltage cannot be obtained in the power supply unit of the display panel 3 when the detection element itself has an obstacle. For example, when leakage takes place in the above-mentioned detection element itself, according to a level of the leakage the above-mentioned forward voltage Vf from the detection element falls, and accordingly an output voltage from the power supply unit for driving the above-mentioned panel falls. Thus, it is impossible to expect normal display operation in the display panel.
In order to cope with such a problem, as described above, it is possible to have a leakage detection means for detecting whether or not the leakage takes place in the detection element, so as to selectively stop the supply of the constant current from the above-mentioned current source I1 to the detection element where the leakage has occurred. By providing the above-mentioned leakage detection means it is possible to stop use of the detection element where the leakage has occurred, whereby a suitable drive voltage can be obtained in the above-mentioned power supply unit.
However, when control is carried out to stop use of the detection element where the leakage has occurred, it is assumed that the above-mentioned leakage detection means for performing the control operation is operating normally. When the above-mentioned leakage detection means does not operate normally, a basic operation of stopping use of the detection element where the leakage has occurred cannot be guaranteed.